Microlenses are required in many applications, such as light coupling from lasers to fibers, either as single lenses or in array form whereby several beams are focused to several fibers. Other important applications include light diffusion and screens.
Depending on the application, one may require a microlens of accurate profile with controlled focusing properties or, in the case of an array, high quality over most lenses in the array. To focus light efficiently, the lens profile (or sag function) must be fabricated with accuracy typically equal to or better than, for example, λ/4, where λ is the wavelength of the illumination source.
In addition, particularly for high-density coupling, diffusion, or screen applications, it is often important that the microlenses utilize the entire surface for focusing. In this way, essentially all incident light can be controlled by the array. When the entire useful surface area is employed for focusing, the array is said to possess a 100% fill factor.
Close packing of microlenses implies a fill factor equal to 100%, which means that the internal boundaries between neighboring microlenses are in close contact. A simple example of close packing is a hexagonal array. Other arrangements, such as square arrays, can also be close packed.
It is typical to find in both the scientific and patent literature arrays of microlenses that have fill factors below 100%. FIG. 1 illustrates such an array where microlenses 12 are regularly placed on the available substrate area 11 with spaces being left between the individual microlenses. One of the unit cells of the array of FIG. 1 is shown by dashed lines 13. The fill factor for this array is only 44%.
There are several existing methods for fabricating isolated microlens units or arrays of microlenses whose edges are well-separated so that their boundaries avoid close contact. Because there is a finite distance between the internal boundaries of neighboring lenses, the fill factor for the array is necessarily less than 1 (or 100%).
The difficulty in obtaining efficient closed-packed lens arrays using prior art fabrication methods is due to the inability of those methods to preserve the boundaries of microlenses accurately, particularly for small and strongly focusing lenses.
Methods using thermal deformation, such as that disclosed in U.S. Pat. No. 5,324,623, are based on volume relaxation and thus cannot control the fusing of material at the internal boundaries between microlenses. With fusion there is distortion that reduces focusing capabilities. Thermal deformation methods are simple to implement but allow limited control of the individual microlens structures.
Other methods, such as those described in U.S. Pat. No. 5,300,263, involve the creation of mechanical molds that define receptacles for curable liquids. The liquid is poured into the receptacles and the natural surface tension creates a bowed surface that serves as the microlenses. The mold, with the various receptacles, defines the array arrangement. Due to the inherent limitation of this method in controlling the shape of the microlens units, its efficiency cannot be optimized for a general application. Other mechanical methods based on the direct ruling of individual microlenses, such as diamond turning, are better suited for the fabrication of individual microlenses rather than arrays.
Methods based on ion diffusion processes that provide gradient-index arrays, such as those described in U.S. Pat. No. 5,867,321, cannot provide a 100% fill factor, with the region between two neighboring microlenses being typically 20% of the microlens repetition spacing. Gradient-index arrays present a serious limitation for large-volume fabrication due to the intrinsically slow diffusion process.
Processes for producing microlens arrays using direct laser writing in a photoresist are known in the art. See commonly-assigned PCT Patent Publication No. WO 99/64929, Gale et al., U.S. Pat. No. 4,464,030, and Micro-Optics: Elements, systems and applications, Hans P. Herzig, ed., Taylor & Francis, Bristol, Pa., 1997, pp. 53–152. The photoresist of choice for such processes is a positive photoresist since compared to negative photoresists, positive photoresists are more widely available, have been subject to more intensive research and development work by photoresist manufacturers, and generally have higher resolution. However, as discussed in detail below, prior to the present invention, it has not been possible to produce arrays of positive microlenses having high focusing efficiencies at high fill factors using positive photoresists.
The present invention addresses the difficulties associated with the prior art by providing methods for fabricating microlens arrays having high focusing efficiencies through accurate microlens fabrication at high fill factors. The array can be arranged in any arbitrary way, such as square, hexagonal, or random. In addition, the methods allow the fabrication of microlenses of arbitrary shape as well as variable focusing power for different directions (anamorphic lenses).